1. Technical Field
Various embodiments of the disclosed technology relate to acoustic delay lines and, more particularly, devices providing bulk acoustic wave delay lines and methods of fabricating such devices.
2. Description of Related Art
Wideband reconfigurable true time delays (TTD) are desired components for a wide variety of high frequency (e.g., GHz-range) applications including phased array radars, electronic filters and countermeasure systems, and digital systems where precise delays are needed to increase effective sampling and processing rates in a large frequency range. However, existing TTD devices are typically expensive, bulky, high loss, or have relatively slow reconfiguring speed.
Acoustic delay lines have been used extensively in signal processing applications to provide TTD. Two common types of acoustic delay line are the surface acoustic wave (SAW) delay line and the bulk acoustic wave (BAW) delay line. Devices providing SAW delay lines are frequency limited by the spacing of the interdigital transducers used to excite the SAW and can be difficult to fabricate in the GHz range as the insertion loss typically increases significantly with frequency.
BAW devices inherently have lower insertion losses as compared with their SAW counterparts but are frequency limited by transducer layer thickness. With early BAW delay line devices, large delays (e.g., 1 microsecond) were desired so thick substrates were used to convey and delay the acoustic waves, as in U.S. Pat. No. 4,099,147These designs suffer from significant diffraction and material losses. Moreover, such designs are not compatible with wafer-scale fabrication processes for low-cost high-volume production.
Modern BAW delay line devices typically rely on long or complicated BAW propagation paths to achieve a desired signal delay. For example, some of these devices are fabricated with angled acoustic reflecting surfaces, as in U.S. Pat. No. 2,672,590Other BAW devices employ a series of two-port delay line devices fabricated on a slanted multifaceted solid to achieve differing delay times among transducers, as in U.S. Pat. No. 2,927,284However, fabricating precise angled acoustic reflecting surfaces and slanted substrates is difficult for high frequency applications. Also, as the number of unique delays required increases, the overall footprint of conventional delay lines increases to accommodate placement of additional devices.